1. Field of Invention
The present invention relates to a semiconductor package structure. More particularly, the present invention relates to a semiconductor package with a higher cooling capability.
2. Description of Related Art
As the integration of an integrated circuit (IC) chip increases, capability and pin count of the IC package also increase correspondingly. In the meantime, the package is more vulnerable to interference by external electromagnetic fields and damages caused by high temperature. Due to an increase in the level of integration, overall volume of an IC package decreases or remains the same despite an increase in pin count. However, a larger pin count for the same package volume means that the pin-to-pin separation has to decrease, and a decrease in pin-to-pin separation renders IC chip packaging more difficult. To resolve the issue, a ball grid array (BGA) type of package structure has been developed. Furthermore, heat generated by the IC chip can be dissipated and unwanted electromagnetic fields can be shielded by including a heat sink in the package.
FIG. 1 is a schematic, cross-sectional view of a conventional BGA package. As shown in FIG. 1, a silicon chip 12 is mounted on a lead frame 10 and is electrically connected to the leads of the lead frame by conductive wires 14. There is a heat sink 16 arching over the lead frame 10, as well. Besides increasing the cooling of the silicon chip 12, the heat sink 16 is also capable of shielding the chip 12 against interference from external electromagnetic fields. The lead frame 10, the silicon chip 12, the conductive wires 14 and a portion of the heat sink 16 are enclosed by placing the lead frame 10 into a mold (not shown) after which a packaging material 20 is injected into the mold. On solidification, the packaging material 20 not only fixes the positions various internal elements, but also provides additional protection against vibration or impact. There is a metallic plate 13 at one corner of the lead frame 10. The metallic plate 13 is located under a gate (not shown), which is an opening through which the packaging material enters the mold. By introducing a smooth metallic plate 13 under the gate, packaging material 20 is able to flow rapidly into the mold cavity. After molding, the mold is dissembled and the package is taken out. Any residual material remaining above the gate can be easily scratched away due to the presence of a smooth metallic surface under the gate.
However, the gate is located at one corner of the packaging cavity inside a mold. Because packaging material 20 has to traverse the cavity from the gate at one corner to the opposite corner, a longer period is needed to fill the entire packaging cavity. Besides slowing the molding process, the longer flow path through this gate arrangement may introduce additional pressure that can result in residual stress inside the package after setting. Furthermore, to secure the heat sink 16, a large section of the heat sink 16 has to be enclosed by the packaging material 20. Since only a portion of the heat sink 16 is exposed, capacity for cooling a silicon chip is greatly reduced.
In brief, the disadvantages of a conventional BGA package include:
1. Since packaging material is injected through a gate at one comer of the packaging cavity, a high amount of residual stress is likely to remain inside the package on solidification. In addition, a longer period is needed to fill the entire packaging cavity. PA1 2. After molding, residual material on the gate has to be removed from the package, thereby incurring addition cost.